Field
The disclosed embodiments relate to batteries for portable electronic devices. More specifically, the disclosed embodiments relate to techniques for performing adaptive effective C-rate charging of batteries for portable electronic devices.
Related Art
Rechargeable lithium-ion batteries are used to provide power in a wide variety of systems, including laptop computers, cell phones, cordless power tools, and electric vehicles. A typical lithium-ion battery cell design includes a porous graphite electrode, a polymer separator impregnated with electrolyte, and a porous cobalt dioxide electrode. The details of the transport of lithium and lithium ions in and out of the electrode granules and through the material between them are complex, but the net effect is dominated by a slow diffusion process for filling one electrode with lithium and another diffusion process for removing lithium from the other.
During charging, the slow diffusion causes a transient build-up of lithium at the surface of the graphite electrode that varies directly with the charging current and a diffusion time that depends upon various environmental, design, and construction factors. If the concentration of lithium at the graphite surface reaches the saturation concentration for lithium in graphite, more lithium is prevented from entering the graphite electrode until the concentration decreases. A significant challenge in charging lithium-ion batteries is to avoid lithium surface saturation at the graphite electrode, while minimizing the charging time by charging as quickly as possible.
The conventional charging method is called the constant-current constant-voltage (CCCV) method that charges at a constant current until a fixed upper voltage limit (e.g., 4.2 V) is reached, and continues to charge by holding the voltage limit constant until the current tapers to a cutoff limit. It is common in the field to express all currents in terms of the cell capacity. For a cell with a maximum capacity Qmax of 2500 mA·hr, a “1 C” current would be 2500 mA, where the unit C or C-rate is a capacity expressed current in units of 1/hr to be multiplied by Qmax to get the current in amps. In the example shown in FIG. 1, the constant current setting is 0.28 C, and the constant voltage phase at 4.2 V is terminated when the current decreases to 0.05 C, indicating a fully charged battery.
The problem with the CCCV method is that it largely operates blindly as neither the current or the voltage directly correlate with the lithium surface concentration, and neither is adjusted as battery characteristics vary. Consequently, the CCCV profile must assume worst-case variability to avoid saturation, and misses the opportunity to use more current when it is possible to do so.
The Adaptive Surface Concentration Charging (ASCC) method (see U.S. Patent US2009/0259420 by inventors Thomas C. Greening, P. Jeffrey Ungar, and William C. Athas) avoids lithium surface saturation during the charging process by adapting to the dynamics of the lithium transport in a battery through closed-loop control of an estimated single electrode potential (or, equivalently, an estimate of the lithium concentration at the surface of an electrode). FIG. 2 shows an ASCC charging profile, where the battery voltage is servoed to maintain the estimated graphite electrode potential to a target voltage.
The ASCC method, while appropriately adaptive, can charge unnecessarily slowly due to the over-conservative estimate of the graphite electrode potential. The ASCC method also requires characterization of specially made three-electrode cells, typically constructed with an inserted lithium reference electrode near the separator. These three-electrode cells are difficult to manufacture without creating significant differences from the characteristics of the two-electrode cells they are intended to match.
The temperature-dependent multi-step charging method (I-V-T), described in U.S. Patent US2009/0273320 entitled “Controlling Battery Charging Based on Current, Voltage, and Temperature” by inventors P. Jeffrey Ungar, Thomas C. Greening, William C. Athas, J. Douglas Field, and Richard M. Mank, provides a method for obtaining multi-step constant-current constant-voltage charging profiles that avoid lithium surface saturation while attaining near-optimal charging times. FIG. 3 shows the multiple temperature-dependent current and voltage steps of the I-V-T charging profile, along with the graphite electrode potential indicating an unsaturated graphite surface.
Like the CCCV method, the I-V-T method reduces the charging current as the battery's maximum capacity Qmax decreases by charging during the constant current phases with the current measured in capacity-dependent C-rates instead of amps. The I-V-T method also selects a different charging profile based on discrete ranges of the measured temperature. While the I-V-T method does not require characterization of specialized three-electrode cells like the ASCC method, three-electrode cells are useful for obtaining optimized I-V-T target parameters.
While the I-V-T method achieves near optimal charging times without lithium surface saturation for typical cells at the beginning of their life, the I-V-T method does not adapt to atypical or aged cells with diffusion times that are different from the cells used for I-V-T parameter characterization. With discrete temperature ranges for determining the charging profile, the I-V-T method also charges batteries unnecessarily slowly if near the upper end of the temperature range.
Hence, what is needed is a charging method that can charge as quickly as the I-V-T method, while preventing lithium surface saturation by dynamically adapting to changes in the diffusion time caused by temperature, age, and manufacturing variation. What also is needed is a charging method that does not require the characterization of three-electrode cells that are difficult to manufacture with characteristics statistically similar to the two-electrode cells they are intended to match.